1. Field of the Invention
The invention relates to a power source apparatus suitable for use in a high voltage power source or the like of a copying machine of the electrophotographic system or a printer.
2. Related Background Art
Hitherto, a corona charge is used in a copy transfer process to transfer a toner image on a photosensitive drum after completion of the development of a color copying machine or a color printer to a copy transfer paper. A power source is supplied to a corona wire by a constant current power source of about 6 to 9 kV and 0.1 to 1 mA.
As means for generating output voltages of both positive and negative polarities, there is generally used means such that positive and negative power sources are serially connected, the polarity side of a small output and range is set to a fixed output, an output on the opposite polarity side is made variable, thereby covering necessary positive and negative output ranges.
In recent years, the copy transfer process has again been studied in order to improve a color picture quality, and as a result a charging method has been switched from the corona charge to the contact charge.
In a contact charging system, in order to attract the toner on the photosensitive drum to the copy transfer paper, a transfer brush into contact with the back side of the transfer paper through a Mylar film and a high voltage of the polarity opposite to the polarity of the toner is applied to the transfer brush. To transfer the toners of four colors of yellow, magenta, cyan, and black onto the same copy transfer paper, the applying voltage is raised step by step as the number of transfer times increases.
Since the applying voltage for the toner of the fourth color exceeds 10 kV, an adverse influence such as a leakage or the like occurs. To prevent such an influence, the applying voltage is set to both of the positive and negative polarities, thereby reducing the maximum value of the absolute value of the applying voltage into 1/2. Namely, before performing the copy transfer of the first color, a negative high voltage is applied to the transfer paper and a positive high voltage at the level that is almost equal to that of the negative high voltage upon starting is applied for the fourth color.
According to such output voltage generating means of the conventional apparatus as mentioned above, there are problems such that an output range of a power source on the variable output side is remarkably widened and the costs and size increase and the reliability deteriorates.
Since a transfer current essentially contributes to the copy transfer process, it is desirable to stabilize the transfer current by performing a constant current control. However, there is a problem such that a build-up time remarkably increases at the time of switching of the transfer current because of a floating capacity among the brush, transfer drum, transfer paper, and photosensitive drum.
That is, an equivalent circuit of the system when a photosensitive drum 1 is seen from a copy transfer brush 7 shown in FIG. 13 through a copy transfer drum 2 and a copy transfer paper P is expressed by a CR parallel circuit shown in FIG. 14. In FIG. 14, C denotes a floating capacity among the transfer brush 7, transfer drum 2, transfer paper P, and photosensitive drum 1. R indicates a space impedance corresponding to a micro corona current Ir flowing from the transfer brush 7 to the photosensitive drum 1, and E denotes a high voltage power source.
Although the corona current Ir obviously essentially contributes to the copy transfer, since it is impossible to separate C and R, the sum of the corona current Ir and a charge/discharge current Ic (=dVc/dt) of the floating capacity C is detected as a current. Generally, since the transfer current to be set, namely, the corona current Ir is equal to or less than tens of .mu.A, when a change speed of the output voltage of the high voltage power source is raised upon switching of the current, the charge/discharge current Ic is equal to or less than a target value of the constant current, so that there is a problem such that a build-up time and a build-down time of the high voltage power source are limited by the charge/discharge time of the floating capacity C. Consequently, a time which is required until the corona current Ir reaches the target value is remarkably delayed. Eventually, the image formation time cannot help being increased in order to compensate such a delay time.
On the other hand, in a contact copy transfer process of a monochromatic copying machine or printer, the constant current control is executed for a non-image period of time with respect to the current supply of a copy transfer roller (or transfer brush), the applying voltage at this time is measured and stored, a predetermined numerical arithmetic operation is executed to the stored value of the applying voltage, and a voltage which is equal to the result of the arithmetic operation is supplied for an image period of time. As mentioned above, it is necessary to switch the drive mode to either one of the constant current drive mode and the constant voltage drive mode in accordance with the non-image period of time and the image period of time. Moreover, the polarities of the output differ in dependence on the non-image period and the image period.
Hitherto, the high voltage power sources of opposite polarities as variable outputs are selectively supplied as a power source for current supply in accordance with the non-image period and the image period.
In such a conventional apparatus, the output cannot be changed to the positive or negative polarity within the non-image period or the image period. Therefore, there is a case where the stable constant current control or stable constant voltage control cannot be performed in dependence on a latent image potential on the photosensitive drum or the residual charges on the transfer paper.
On the other hand, when switching between the non-image period and the image period, it is necessary to shut off one of the positive and negative high voltage power sources and to build up the other one, so that there is a drawback such that the switching time is long.
Hitherto, there has also been proposed a method whereby the polarities of a transformer of a variable output and a transformer of a fixed output are made different, thereby obtaining positive and negative variable outputs as shown in U.S. Pat. No. 5,164,771. However, since the fixed output type transformer always operates, there is a large vain operation.
Hitherto, various kinds of high voltages are used to form an image of an apparatus of the electrophotographic system. In case of using a copy transfer drum, there is a case of applying a high voltage for a copy transfer to the copy transfer drum itself. In this instance, an adsorption charging device to wrap a copy transfer paper around the copy transfer drum, a separation charging device for separating the transfer paper from the copy transfer drum, a discharging device for discharging the charges on the copy transfer drum, and the like are arranged around the copy transfer drum. Different high voltages are respectively applied to those charging devices. For this purpose, different high voltage power sources are used for the copy transfer drum and the charging devices, respectively.
The power source to produce a voltage which is applied to the copy transfer drum is controlled so as to generate the optimum voltage in accordance with the copy transfer process by the sequence control. In a manner similar to the above, the optimum voltage is also applied to each charging device in accordance with each process by the sequence control.
According to such a conventional apparatus, a voltage such that the voltage that is applied to each charging device effectively functions is a voltage corresponding to a difference between the voltage which is applied to the copy transfer drum and the voltage which is applied to each charging device. Therefore, as a voltage which is applied to each charging device, the voltage to which the voltage that is applied to the copy transfer drum was multiplexed must be produced. Consequently, there are inconveniences such that both of the circuit and the control sequence are complicated and a voltage precision cannot be easily improved.